'All of their arguments have broken down into naked patriarchy, and that really isn’t an attractive sight, as the story of Noah makes clear' … Detail from the Cappella Palatina, Palermo. Photograph: Andrew Brown

When in a hole, stop digging. No one seems to have explained this simple principle to the evangelical opponents of gay marriage, with the result that their increasingly hysterical campaign is looking less and less convincing to the world outside their bubble.

In the runup to tomorrow's vote in the Lords, I have had three press releases from Christian Concern, the pressure group which was for 10 years or more the main source of stories on how Christians were being persecuted by the secular state, and was the main backer of the various lawsuits against gay equality which have all now failed.

The first was an account of a demonstration held outside Buckingham Palace last Sunday. Originally, this was going to involve a child handing over a letter for the Queen urging her "to please do all in Her Majesty's power to stop this bill from becoming law. Members of the House of Lords will vote on the bill on Tuesday 4 June after its second reading. Would Her Majesty join with many other Christians and pray that Her Majesty's noble Lords would reject this proposed departure from God's holy and good design for marriage?"

This seems to have been dropped in favour of a sermon from Bishop Michael Nazir Ali to a crowd whom I counted, from their photograph, to have been 56 adults and a dozen children, though there are much larger numbers of tourists in the background who might, with the eye of faith, be seen as lending their support. Nazir Ali's argument that gay marriage breaches her coronation oath to uphold the Protestant religion is the kind of silly cleverness that marks a conservatism detached from the shackles of reality.

Then there was a letter urging the archbishop of Canterbury to "whip in" his bishops to the House of Lords to vote against the bill. It was signed by about 30 independent evangelical pastors. At least half were Africans. I'm sure they'd all be delighted if the archbishop of Canterbury were to offer them public and unsolicited advice on how to run their churches in return for theirs.

More to the point is the fact that it argues – I think correctly – that this bill will lead to a weakening of the status of the established church and a diminution of the role of Christianity in the national life. The trouble is that its supposed remedy – even if it were possible, which it is not, since some bishops would vote in favour of the bill – would make both these problems worse. If there is one thing that I have learned from the comments on this site in the last four years, it is that homophobia breeds hatred in return. A really significant proportion of the commenters who hate and despise Christianity do so because they feel that Christians stand for the hatred and contempt of gay people.

The Church of England has – 15 years too late – learned this lesson too. This morning's Daily Telegraph claims that bishops opposed to the measure are being urged not to turn up and vote lest it seem that the church is voting as a bloc against the wishes of the nation.

By pure coincidence, I spent part of this morning in a radio studio, for a discussion hosted by the evangelical Premier Radio on whether Christians are marginalised in this country. My role was to inject a note of sour realism by pointing out that conservative evangelicals (for that is whom they mean by "Christians") had largely marginalised themselves and no one else felt sorry for them. What has set them against mainstream culture is not some kind of timeless spirituality but their own very time-bound subculture.

This started as an argumentative position but hardened into a conviction of truth by the end of the discussion. The fiasco over gay marriage is part of a general defeat for conservative Christians right now. The other wing, often involving the same people, is the collapse of resistance to women bishops in the Church of England. When I read that Conservative evangelicals are outraged at the prospect of admitting that women are lawfully bishops and their superiors and feel that they will have to lie or leave the church, as a recent press release stated, I want to break out the world's smallest violin and play a jig on it.

All of their arguments have broken down into naked patriarchy, and that really isn't an attractive sight, as the story of Noah makes clear.

There will always be conservative Christians, of course; and there will always be silly liberal policies which they are right to resist. But for the foreseeable future there won't be any credible conservative Christian organisations to voice their fears.